The present invention relates to a process for making water soluble or water swellable polymers, by polymerisation of water soluble ethylenically unsaturated monomer or monomer blend. In particular the invention relates to processes of making said polymers containing low concentrations of residual monomer.
Water soluble and water swellable polymers are used in numerous industrial applications, for instance, flocculants, coagulants, rheology modifiers, dispersants, superabsorbents and binders. Of particular importance are high molecular weight water soluble polymeric flocculants which may be used as retention or drainage aids in paper making or to flocculate sludges such as sewage sludge, waste waters, textile industry effluents red mud from the Bayer Alumina process and suspensions of coal tailings etc.
It is standard practice to prepare water soluble or water swellable polymers by polymerising water soluble monomers using a suitable initiator system. The polymers are usually provided either as a solid particulate product or as a reverse phase dispersion or emulsion. Typically particulate polymers are prepared introducing initiators into an aqueous solution of the monomers and polymerising to form a polymer gel which is then cut into smaller pieces, dried and then ground to the appropriate particle size. Alternatively the polymers are produced as beads by suspension polymerisation or as a water-in-oil emulsion or dispersion by water-in-oil emulsion polymerisation, for example according to a process defined by EP-A-150933, EP-A-102760 or EP-A-126528.
It is known to produce water soluble and water swellable polymers using a variety of initiator systems. For instance it is common practice to polymerise water soluble monomers using redox initiator couples, in which radicals are generated by admixing with the monomer a redox couple which is a reducing agent and an oxidising agent. It is also conventional practice to use either alone or in combination with other initiator systems a thermal initiator, which would include any suitable initiator compound that releases radicals at an elevated temperature. Other initiator systems include photo and radiation induced initiator systems, which require exposure to radiation to release radicals thereby effecting polymerisation. Other initiator systems are well known and well documented in the literature.
Although water soluble and water swellable polymers can be prepared using many of the commercially available initiator systems, it is often difficult to prepare on an industrial scale polymers which have the correct molecular weight in combination with other desired characteristics, such as solubility, degree of absorbency etc. Over the last ten to fifteen years it has also become increasingly important to provide polymers which have extremely low levels of residual free monomer. This is particularly the case for polymers based on acrylamide monomer.
There have been various proposals in the literature for reducing residual free monomer concentrations in polymers, especially polymers of acrylamide. For instance in U.S. Pat. Nos. 4,906,732 and 4,996,251 polyacrylamides are treated with an amidase enzyme which is active towards acrylamide. However, although it was possible to achieve very low levels of free acrylamide, the enzymes proposed in these patents cannot consistently especially at elevated temperatures.
WO-A-97 29136 describes an amidase enzyme which is particularly effective at high temperatures and thus can be applied to the hot polymer gel substantially immediately prior to the drying stage. However, although this enzyme has shown particular advantages over other known amidases, it is still nonetheless difficult to consistently achieve low residual levels of acrylamide on an industrial scale.
Therefore there exists a need to be able to conveniently and consistently provide water soluble or swellable polymers with no or extremely low levels of residual monomer, especially acrylamide monomer.
There also exists a need to achieve this in an industrial scale process and in particular in a process which does not require additional long residence stages in the production process.